In the note, AT&T blames a spectrum shortage for the policy, and says the only option is to complete its merger with T-Mobile.

The new policy starts October 1. For customers who still have AT&T's unlimited data plan, your download speed may be reduced once you hit the threshold considered to be in the top 5% of data usage for one billing period.

AT&T did not say what that threshold is. However, AT&T says it will warn you before it's about to stop throttling your data speeds and give you a grace period before it starts doing so.

If AT&T does reduce your data speed, it will return to normal at the beginning of the next billing period.

The policy will not affect users who subscribe to one of AT&T's current tiered data plans. The only customers who have unlimited data plans are those whose contract were grandfathered before the tiered plans began.

The timing is just a few weeks after the iPhone 5's expected launch in mid-September. AT&T is also expected to launch its faster LTE network this fall, which will put even more strain on the network.